1. Field of the Invention
The invention relates to an apparatus and a method for controlling transmission and/or reception of signals in a radio communication system, especially for application in base stations of mobile radio communications systems.
2. Description of the Related Art
In radio communications systems, signals are exchanged between radio terminals and base stations via a so called radio interface or air interface. Radio terminals are in general mobile or stationary user terminals (UE—user equipments), whereas base stations (NB—Node B) are access stations associated with a land based communication network. Examples of known radio communication systems are second generation digital mobile radio communications systems like GSM (Global System for Mobile Communication) based on TDMA (Time Division Multiple Access) and providing data rates up to 100 kbit/s, and third generation digital mobile radio communications systems like UMTS (Universal Mobile Telecommunication System) based on CDMA (Code Division Multiple Access) with data rates up to 2 Mbit/s.
Antenna arrays can be used in any type of system that transmits and/or receives radio frequency signals using one or a plurality of antennas. The use of antenna arrays in above described systems provides for antenna performance improvements over the use of a single element antenna, including improved directionality, signal to noise ratio and interference rejection for received signals, as well as improved directionality, security, and reduced power requirements for transmitted signals. Antenna arrays can be used for signal reception only, for signal transmission only, or for both signal reception and transmission. Most antenna array systems consist of an antenna array and a signal processor that processes the signals going to and coming from individual array elements.
Thus, an antenna array is composed of a number of so called antenna elements, each connected to a radio frequency (RF) transceiver (transmitter/receiver). In reception mode, the receivers obtain RF signals from each antenna element and apply a down conversion of the received signals to base band signals. In the base band, demodulated signals are then compared with each other in amplitude and phase. The information on the direction of arrival (DOA) of the incoming signal, i.e. the direction of the transmitting station, is contained in the relationship between the received signals. In transmission mode, this information is subsequently used for beamforming (BF) in the direction of the received signal by correctly weighting base band signals for the different transmitters connected to the antenna elements.
The procedure described above can only be realized with a certain accuracy if the characteristics of the individual transmitters and receivers are known, so that these characteristics can be taken into account for the DOA and BF algorithms. To be precise, transfer functions (in amplitude and phase) from antenna elements to the base band outputs of the receivers as well as transfer functions from the base band inputs of the transmitters to the antenna elements must be known. During operation these transfer functions are subject to parameter variations (drift) of active and passive elements in the transceivers and cables. Therefore, transfer functions have to be continuously or at least periodically determined during operation of the transceivers.
Two different approaches of calibration procedures are known in the art. According to a first procedure, a known signal is fed to a test antenna (calibration antenna) which is arranged close to or as part of the antenna array (known from U.S. Pat. No. 6,236,839) or is separated from the antenna array (known from U.S. Pat. No. 5,546,090). The base band signals carry information about the transfer functions of the individual receiver paths, which can then be compared and adjusted. This procedure is called RX calibration.
According to a second procedure, known signals are fed to the individual antenna elements and received by a test antenna. The test antenna could thereby be located as described above.
The received signals carry the information about the individual transfer functions of the transmitter paths, which are subsequently compared and adjusted. This procedure is called TX calibration.
Both calibration procedures can be realized either simultaneously, which is a preferred solution in systems using frequency division duplex (FDD), or consecutively as preferred in systems using time division duplex (TDD).
Configurations enabling the above described procedures are shown in FIGS. 2 and 3. According to these configurations, a calibration antenna is connected by a duplexer or switch to calibration transmitter (TXc) and receiver (RXc) circuits operating in the radio frequency range. Signals from/to the calibration antenna are processed in a calibration processor operating in the base band. The calibration processor is connected to a beamforming processor that processes signals from/to the individual antenna elements (#1 . . . #n) of an antenna array of e.g. a base station. Coefficients representing the determined variations are stored in lookup tables.
In a RX calibration procedure (FIG. 2, the signal flow is presented by broken lines), the calibration processor initiates the transmission of test signals from the calibration antenna over the air interface to the individual antenna elements of the antenna array. The received test signals are then fed back to the calibration processor by the beamforming processor. Within the calibration processor, transfer functions of the individual receiver paths are determined and evaluated and stored in a lookup table in order to be taken into account for normal operation of determining directions of arrival.
In a TX calibration procedure (FIG. 3, the signal flow is presented by broken lines), the calibration processor initiates the transmission of test signals from each of the antenna elements which are received by the calibration antenna. The received signals containing information about the transfer functions of the individual transmitter paths are then evaluated in the calibration processor and stored in a lookup table in order to be taken into account for the normal operation of beamforming.
The described procedures suffer from the fact that specialized calibration means have to be integrated within each base station, thereby causing additional costs and space requirements.
It is therefore an object of the invention to provide calibration arrangements which do no suffer from the above stated disadvantages.